Many medical procedures are performed using minimally invasive surgical techniques where one or more slender implements are inserted through a small incision into a patient's body. Minimally, invasive surgical implements for ablating tissue can include a rigid or flexible structure having an ablation device at or near its distal end that is placed adjacent to the tissue to be ablated.
There are many procedures that include ablating certain tissue. For example, cardiac arrhythmias can be treated through selective ablation of cardiac tissue to eliminate the source of the arrhythmia. One type of minimally invasive procedure includes the use of an ablation catheter subsequent to a preliminary step of electrocardiographic mapping. After examination of the mapping results, one or more ablated regions (lesions) are created in the cardiac tissue.
A number of cooled catheter systems (cryocatheters) have been developed for treating tissue in a cardiac setting, either to cool the tissue sufficiently to stun it and allow cold mapping of the heart and/or confirmation of catheter position with respect to localized tissue lesions, or to apply a more severe level of cold to ablate tissue at the site of the catheter ending. In general, the range of treatments which may be effected by a cryocatheter is comparable to the range of applications for RF or thermal ablation catheters, and in particular, these instruments may be configured to achieve either small localized ball shape lesions at the tip of the catheter, or one or more elongated linear lesions extending a length of several centimeters or more along the tip. Elongate lesions are commonly used to achieve conduction block across a region of the cardiac wall so as to sever a re-entrant pathway, thereby preventing conduction across the region, in order change the cardiac signal path topology. For example, it may be desired to eliminate a re-entrant pathway responsible for atrial fibrillation or a tachycardia.
In general, when used for endovascular access to treat the cardiac wall, for example, catheters of this type must meet fairly demanding limitations regarding their size, flexibility, strength, electrical conductivity and the like which affect their safety. These constraints generally require that the catheter be no larger than several millimeters in diameter so as to pass through the vascular system of the patient to the heart. Thus, any electrodes (in the case of mapping or RF/electrothermal ablation catheters), and any coolant passages (in the case of cryocatheters) must fit within a catheter body of small size.
In addition, there are important safety considerations when using cryogenic catheters for non-invasive procedures. For example, the cryogenic fluid used to cool the catheter tip may leak so as to enter the patient's body. Further, a vacuum used to exhaust spent fluid may remove blood from the patient into the fluid recovery reservoir. In addition, a particular procedure may have to be aborted prematurely without achieving the desired therapeutic effect if the cryocatheter system has insufficient coolant.
Furthermore, it may be desirable to treat tissue using a predetermined time and temperature schedule. However, manually timing the length of a procedure and repeatedly adjusting the tip temperature can lead to operator error, as well as inefficient treatment of the tissue. That is, the applied cryogenic energy may not be applied so as to maximize tissue destruction. In addition, the actual tip temperature may be different than a selected temperature due to thermal variations at the treatment site.
It would, therefore, be desirable to provide a cryogenic catheter system that controls and monitors operating parameters, automatically if desired, to achieve safe and effective cryogenic treatment of tissue.